This invention relates generally to nuclear reactors and more particularly, to apparatus for repairing jet pump assemblies within a nuclear reactor pressure vessel.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The purpose of the gravity actuated wedge is to maintain contact between the inlet mixer and the restrainer bracket. The wedge works in cooperation with two set screws which are tack welded to the restrainer bracket to maintain contact with the inlet mixer. The flow of water through the jet pumps typically includes pressure fluctuations that are caused by various sources in the reactor system. The pressure fluctuations can have frequencies close to one or more natural vibration modes of the jet pump piping. The jet pump piping stability depends on the tight fit-up, or contact, of the restrainer brackets and the inlet mixers. Operating thermal gradients, hydraulic loads, and fluctuations in the hydraulic loads can overcome the lateral support provided by the gravity wedge, allowing gaps or clearances to develop at the opposing two fixed contacts or set screws. Alternately, the set screw contacts may develop excessive clearances from wear or by damage from failure of other support components, such as the inlet mixer hold down beam. The loss of contact between the inlet mixer and the restrainer bracket can change the jet pump natural frequency to match some excitation frequency in the system, causing vibration of the piping and exerting increased loads which may cause cyclic fatigue cracking and wear of the piping supports, which can result in degradation from wear and fatigue at additional jet pump structural supports. When such conditions are discovered during plant shutdown, restoration of the tight fitup at the set screw contact locations must be accomplished by apparatus that can be fabricated and installed in a minimum time to avoid extension of the repair outage.
To overcome this problem, gravity wedge supports have been previously used at locations where clearances have developed in restrainer bracket contacts. The gravity wedge support employed a sliding wedge and a fixed bracket mount that engaged the jet pump restrainer bracket. These gravity wedges were only applicable to restrainer bracket/inlet mixer gap widths from about 1.0 to 2.0 inches, as space was required for a wedge with sufficient weight to give the desired support load. Another solution employed spring wedge supports at locations where clearances have developed and the restrainer bracket design did not allow adequate access space for a gravity actuated wedge. Both these types of wedges required significant fabrication, which had to be performed in advance of the installation to avoid extension of the reactor outage length. Also these wedges made use of guide ear and gusset features of the restrainer bracket for locating the wedge. However, guide ear and gusset features are not present on restrainer bracket designs for some plants.
In one aspect, a piping support wedge apparatus for a jet pump in a nuclear reactor is provided. The wedge apparatus includes a first tapered wedge segment having a first end portion and a second end portion and a second tapered wedge segment having a first end portion and a second end portion. The first and second wedge segments are joined at the first ends portions to form a substantially U-shaped body. The wedge apparatus also includes a slot defined by an area between the first and second wedge segments and extending from the first end portions to the joined second end portions of the wedge segments.
In another aspect, a nuclear reactor jet pump assembly is provided that includes an inlet mixer, a diffuser coupled to the inlet mixer by a slip joint, a restrainer bracket having at least one set screw engagable with said inlet mixer, and a piping support wedge apparatus positioned between the restrainer bracket and the inlet mixer. The wedge apparatus includes a first tapered wedge segment having a first end portion and a second end portion and a second tapered wedge segment having a first end portion and a second end portion. The first and second wedge segments are joined at the first ends portions to form a substantially U-shaped body. The wedge apparatus also includes a slot defined by an area between the first and second wedge segments and extending from the first end portions to the joined second end portions of the wedge segments.
In another aspect, a method of restoring a tight rigid fit between an inlet mixer and an adjacent restrainer bracket in a nuclear reactor pressure vessel is provided. The restrainer bracket is coupled to a jet pump riser pipe and includes at least one set screw. The method includes inserting a piping support wedge apparatus between the restrainer bracket and the inlet mixer, and compressing the wedge apparatus around the restrainer bracket set screw. The wedge apparatus includes a first tapered wedge segment having a first end portion and a second end portion and a second tapered wedge segment having a first end portion and a second end portion. The first and second wedge segments are joined at the first ends portions to form a substantially U-shaped body. The wedge apparatus also includes a slot defined by an area between the first and second wedge segments and extending from the first end portions to the joined second end portions of the wedge segments.